In the coal industry, it is a common expedient to form long, generally horizontal boreholes in coal-bearing formations in order to provide for the degasification of the coal formations prior to conducting mining activities. After drilling the boreholes, which are generally horizontal, i.e. generally follow the dip of the coal formation, they are vented to a suitable disposal or collection facility in order to remove hydrocarbon gas (methane) from the coal bed. It is usually desirable to drill relatively long gas-relief boreholes since the methane flow rates are directly proportional to their lengths. One difficulty encountered in drilling such holes is in maintaining the bit trajectory within the desired confines of the coal bed.
The systems employed in drilling gas-relief boreholes fall generally into two categories. In one case, the drill bit is rotated by means of a downhole drill motor which is either electrically or hydraulically powered. In the other case, the drill bit is driven by means of a rotating drill string which is connected to a suitable power source, such as a power swivel, located externally of the hole.
One useful system for controlling the directional advance of a hole drilled wherein power is supplied by means of a downhole drill motor is disclosed in U.S. Pat. No. 3,888,319 to Bourne et al. This system comprises a thrust unit, a roll control unit, a deflection unit, and the downhole drill motor. The downhole thrust unit includes a hold unit and a retraction unit, each comprising pressure feet which are adapted to be hydraulically forced against the side of the borehole to anchor the unit in place. The deflection unit comprises diametrically opposed pistons which are coupled together and are actuated by fluid pressure against the bias of a compression string to deflect the drill bit in one of two opposite directions. The roll control unit functions to orient the assembly at any desired angular displacement within the borehole. In operation, the assembly is anchored in the hole by means of the thrust unit and the drill bit advanced as it is rotated by the motor relative to the thrust unit. The orientation of the deflection unit is controlled by the roll control unit so that the appropriate piston can be biased against the side of the borehole to deflect the drill bit from the axis of the hole.
Bourne et al disclose various arrangements of the system components. In most cases, the deflection unit is located adjacent the drill bit. However, an alternative arrangement disclosed in Bourne et al involves locating a stabilizer adjacent the drill bit to centralize the assembly within the borehole. The deflection unit is then located behind the stabilizer so that actuation of the deflection unit causes the drill bit to pivot about the stabilizer. An alternative system disclosed in Bourne et al involves the use of a system having an external roll and power control unit which is connected to the downhole assembly by means of hollow drill pipes. The drill string is supported in the hole by means of a pair of longitudinally spaced stabilizers. The instrument package, drill motor and deflection unit are located between the front stabilizer and the drill bit.
The use of rotary drill string systems to drill gas relief holes is disclosed in Cervik, Joseph, et al. "Rotary Drilling Holes in Coal Beds for Degasification", Bureau of Mines Report of Investigation 8097, United States Department of the Interior, 1975. This report describes the use of various centralizer (stabilizer) configurations in combination with bit thrust and bit rotational velocity to control the bit trajectory. Thus, the authors disclose that by placing a short centralizer immediately behind the bit, the hole will follow a slight upward arc under the appropriate conditions of thrust on the drill string and rotational speed. By locating the centralizer about 10 feet behind the bit, the weight of the drill pipe and bit in front of the centralizer bends the drill string slightly downward and, consequently, the hole follows a curved path downward. Cervik et al also disclose the use of two centralizers, one directly behind the bit and the other spaced 10 to 20 feet behind the first centralizer. In this case, the borehole also follows a slightly downward path.